This invention relates to radar antennas and particularly to circular radar antenna arrays and the feed networks therefor.
In the usual air traffic management system a ground radar station transmits an interrogation message throughout its sphere of interest. A transponder equipped aircraft operating within the sphere of interest and receiving the interrogation message automatically transmits a response message whose exact format depends upon the exact format of the interrogation message. More particularly, the ground station transmits the interrogation message along a narrow beam into the sphere of interest. The direction from the ground station of an aircraft whose response is received at the ground station is known since the responding aircraft must normally be within the narrow beam in order to be interrogated and to thus respond. A coding scheme is used to ensure that aircraft which are not within the narrow beam do not respond to the interrogation message carried on the narrow beam side lobes. The coding scheme provides that the initial portion of the interrogation message consists of three coded pulses, designated P1, P2 and P3 transmitted by the ground station in that order. Pulses P1 and P3 are transmitted only on the narrow beam and pointed in the specific predetermined direction from the ground station, while pulse P2 is transmitted omnidirectionally. As a result, an aircraft within the narrow beam hears pulses P1 and P3 of relatively high amplitude and pulse P2 of relatively low amplitude and aircraft outside the narrow beam perceive pulses P1 and P3 of relatively low amplitude. The aircraft transponder includes decoding circuits which recognize the aforesaid pulse coding to allow only aircraft within the narrow beam to respond.
Those aircraft which are outside the narrow beam and which receive the P1 and P2 pulses, where P2 is of greater amplitude than P1 will be suppressed, that is, they will not respond during a short predetermined time thereafter even though they may be interrogated during that time by the proper interrogation message. This interrogation message which a transponder receives during its suppression period might, for example, be transmitted from a second, further removed, ground station whose sphere of interest should not extend into this sphere of interest of the first mentioned ground station but which because of atmospheric or siting problems now does. It can be seen that should a transponder respond to interrogation from said second ground station the first ground station will interpret the response erroneously, that is, it will interpret that response as being indicative of an aircraft in the pointing direction of its narrow beam which, in this case, of course, the responding aircraft is not. It is thus important that an aircraft located within the sphere of interest of a particular ground station have its transponder actively suppressed whenever it is out of the main narrow beam of that ground station.
As might be expected, it is also important that the interrogation beam of each ground station be as narrow as possible for good target resolution, that is, to permit different aircraft within a particular sphere of interest but closely spaced in azimuth with respect to the ground station to be individually interrogated.